The manufacture of nonlinear carbonaceous fibers having a reversible deflection ratio of greater than 1.2:1 and derived from a polymeric precursor composition, such as polyacrylonitrile (PAN), is known and disclosed in U.S. Pat. No. 4,837,076 to McCullough et al. In the prior patent, the polymeric precursor material is spun into fibers and collected into multifiber assemblies, such as fiber tows, containing more than 1000 (1 K) individual fibers, but generally not more than 12,000 to 20,000 fibers. The fiber tows are thereafter oxidatively stabilized and formed into a knitted fabric which is then heat treated in a non-oxidizing atmosphere while the fibers are in a relaxed and unstressed condition. Heat treating the fibers increases the carbon content to form carbonaceous fibers which are substantially heat set. The fabric is then deknitted to form non-linear fiber tows in which the individual fibers are characterized by having a reversible deflection ratio of greater than 1.2:1, typically from about 2.5:1 to about 4:1. The fiber tows can then be further processed, as by carding, to form a wool like fluff.
These prior non-linear carbonaceous fibers, although they exhibit substantial advantages in processability over other carbonaceous fibers, still suffer from the disadvantage of having to be knitted from a relatively small tow of fibers of from about 1000 individual fibers (1 K) to about 12000 fibers (12 K) and then having to be deknitted.
Firstly, knitting of the fiber tows and the subsequent deknitting procedure is costly in terms of the equipment that must be used to process the fibers into a wool like fluff and the time that is consumed in knitting/deknitting and subsequent chopping and opening of the fibers to form a wool like fluff.
Secondly, present day knitting machines are not able to accommodate fiber tows that are much larger than 20,000 fibers. Thus, the use of relatively small fiber tows is in itself a limiting factor when it is desired to produce large amounts of the wool like fluff. Another disadvantage of knitting fiber tows is dictated by the fact that the knitting hooks of a knitting machine are not able to engage tows that are larger than 20,000 fibers. The knitting hooks are not large enough to engage the entire tow in its formation of a loop thereby leaving some portion of the fibers of the tow outside of the hook which inevitably leads to a severe entanglement of the fibers during the knitting opperation and eventually to a complete breakdown of the operation.
A further disadvantage of knitting relatively small fiber tows (of less than 20 K) is that the tow of fibers is still so large in diameter that it is not possible to provide the fibers with a fine crimp, as herein after defined, due to the large loops that are formed in knitting a cloth. Specifically, with a 12 K tow, the individual fibers have a relatively large amplitude and a relatively low frequency so that, in effect, the fibers have a reversible deflection ratio of greater than 1.2:1. Accordingly, it is not possible to use the knit/deknit process to produce fibers having a greater number of crimps per inch i.e. on the order of greater than 4 crimps per inch (2 crimps per cm) to provide the fibers with a reversible deflection ratio of less than 1.2:1.
Another drawback of the non-linear carbonaceous fibers of the prior art is that it has not been possible to spin these carbonaceous into fine yarn with acceptable staple length especially when they are blended with other synthetic or natural fibers due to the nature of their crimps. Although the crimps in the fiber are necessary for good processability, the relatively large amplitude and low frequency of the crimps causes excessive fiber breakage during carding and drawing. In addition, the prior art fibers exhibit a fairly low cohesiveness and compromise elongatability to improve tenacity.
As a result of the relatively low cohesiveness of the fibers with a reversible deflection ratio of greater than 1.2:1, any yarn which is produced therewith has a greater bulkiness and is difficult to process into garments without the use of special techniques or apparatus.